1. Field of Invention
The present invention relates to a graphite electrode having joints, used for a steelmaking electric furnace. More particularly, the present invention relates to a graphite electrode, in which current conduction through a joint of the electrode sections is decreased to suppress the heat generation at the joint and hence to prevent a nipple from rupturing.
2. Description of Related Arts
In order to join the graphite-electrode sections used for an electric furnace, a nipple with grooved screw is generally used. This nipple 3 is screwed into the socket 4 of the graphite-electrode sections 1 and 2, as is shown in FIG. 1.
Along with advancement of high-load operation of steelmaking electric furnaces, the graphite electrode and its joint parts are exposed to high electric and mechanical load. Rupturing of the graphite electrode, which may be hereinafter simply referred to as an electrode, therefore occurs frequently, mostly in the joints.
As is known, the temperature of the central portion of an electrode becomes higher in recent direct-current electric steelmaking furnaces than in conventional alternating-current electric steelmaking furnaces. The nipple, which is located in the central portion of an electrode, is therefore exposed to a high temperature, with the result that the rupture is likely to occur.
It is now described how the rupturing occurs with reference to FIG. 2 which illustrates jointed electrode-sections. In FIG. 2, the three electrode sections are joined at the joints "a" and "b" and are integrally suspended by the holder arm 5. As compared with joint "a", joint "b" is subjected to higher mechanical load of the electrode-sections and to higher lateral mechanical load due to raw materials and the like, which strike the tip end of the electrode and generate the bending moment. Although these loads are high at joint "a", the ruptures overwhelmingly much occur in the nipple of joint "a". In addition, no matter how the outer circumferential surface of an electrode wears due to oxidation and is hence weakened, the rupture occurs not in the electrode sections but in the nipple. It seems therefore that a major reason of the rupture lies in thermal load rather than the mechanical load.
It has been heretofore proposed to prevent the joint portion of an electrode from rupturing, by means of modifying the shape of the nipple and screw so as to mitigate the stress concentration on these parts, as seen in Japanese Examined Utility Model Publications No. 57-45,676 and No. 58-958, by means of sandwiching conductive material between the bottom of the socket and the end of the nipple so as to prevent the generation of local current (Japanese Examined Utility Model Publication No. 63-36,639), or by means of modifying the structure of the nipple so as to easily absorb its thermal expansion (Japanese Examined Utility Model Publication No. 59-1,357).
The previous measures for preventing the electrode from rupturing lie in variously modifying the nipple shape or sandwiching the conductive material as described above, but do not intend to decrease the current passing across the nipple. Since the temperature of a nipple is particularly high in the direct-current electric furnace at present, the known methods cannot attain a satisfactory result.
In the direct-current electric furnace with, for example a 30 inch electrode, the current across the nipple and the current across the contact part between the electrode sections is allegedly 50% each, based on the area ratio of the former and latter. However, since the nipple is screwed into and fastened by the socket, the density of the nipple is increased resulting in the decrease of resistivity. The current is therefore more liable to conduct through the nipple than the contact part between the electrode sections.